In an internal combustion engine, the higher the compression ratio, the more the thermal efficiency is improved, while the thinner the air-fuel ratio of the air-fuel mixture, the more the fuel consumption is improved. In a conventional spark ignition combustion internal combustion engine which uses a spark plug to ignite part of the air-fuel mixture and uses propagation of the ignited flame to burn the remaining air-fuel mixture, the compression ratio cannot be raised because knocking would occur. Further, the air-fuel ratio of the air-fuel mixture cannot be made thinner because the ignited flame can no longer propagate. As opposed to this, autoignition combustion is possible even if raising the compression ratio. Further, autoignition combustion is possible even if making the air-fuel ratio of the air-fuel mixture thinner. Therefore, if performing autoignition combustion, it is possible to improve the thermal efficiency and possible to improve the fuel consumption.
Further, in this autoignition combustion, the fuel diffused in a combustion chamber is made to burn in the combustion chamber at many points simultaneously. If the diffused fuel is made to burn simultaneously at many points in this way, the combustion temperature becomes lower over the entire area in the combustion chamber, so the production of NOx is suppressed and, furthermore, sufficient oxygen is present around the fuel, so the production of unburned HC is also suppressed. In this way, autoignition combustion has numerous advantages, so has been drawing attention since the past. Numerous internal combustion engines designed to perform autoignition combustion are known (for example, see Japanese Patent Publication No. 2011-153562A).